We discuss apparent characteristic features of Rayleigh scattering of elastic waves from cracks. Interpreting these features, we propose a procedure that in some experimental situations may be useful to distinguish generally-shaped cracks from volume defects. For elliptically-shaped cracks, we propose additional procedures that in principle allow the unique specification of crack size, shape and orientation; however, we suggest that in practice only the crack plane orientation and a lower bound on the crack length is measurable. We also comment upon the inversion procedure of Kahn and Rice.